The present invention generally relates to articles that benefit from ultrasonic inspection, as well as ultrasonic inspection techniques and systems. More particularly, this invention relates to the ultrasonic inspection of surfaces of axially-oriented slots formed in an article adapted for use in rotating machinery, such as the turbine wheels and rotors of steam and gas turbines.
Ultrasonic inspection techniques have been widely used to perform nondestructive testing on various articles, including those formed of materials with intrinsically coarse grain structures that result in anisotropic and nonuniform acoustic properties. Nonlimiting examples of such articles include forged superalloy turbine wheels (rotors) used in steam and gas turbines. In the hostile operating environments of gas and steam turbines, the structural integrity of a turbine wheel is of great importance in view of the high mechanical stresses that wheels must be able to continuously withstand at extremely high temperatures and rotational speeds.
Ultrasonic inspection techniques employed with turbine wheels have typically involved inspecting the wheel from a plane perpendicular to the highest operating stresses. A typical approach is to place ultrasonic transducers on the fore and/or aft wheel surfaces transverse to the wheel rotational axis. With this approach, ultrasonic energy is generated in a direction substantially perpendicular to the orientation of the most common defects, which tend to lie in axial-radial planes parallel to the fore and aft surfaces of a turbine wheel. Two ultrasonic testing techniques are widely used. The first is a “pitch-catch” technique using two transducers placed on the fore and aft surfaces of the wheel. One of the transducers serves to generate an ultrasonic signal, and ultrasonic signals reflected from acoustical discontinuities are received by the second transducer. The second technique is referred to as “pulse-echo” and makes use of a single transducer to both generate the ultrasonic signal and receive reflected signals.
As well known in the art, the connections that secure turbine buckets (blades) to a turbine wheel are particularly stressed during turbine operation. Such connections are often in the form of complementary retention features defined on the wheel circumference and the roots of the buckets. These retention features, commonly referred to as dovetails, have been used in several different forms. Radial-entry and tangential-entry dovetails are represented in commonly-assigned U.S. Pat. Nos. 6,049,979 and 6,821,086, respectively, and entail one or more male dovetail features that circumferentially extend around the outer periphery of a wheel, and assemble with a complementary female dovetail slot on each bucket. A third dovetail type is the axial-entry dovetail, represented in commonly-assigned U.S. Pat. No. 6,814,543. Axial-entry dovetail connections utilize axially-oriented female slots defined in the wheel circumference, into which a male dovetail of a bucket is inserted in the axial direction of the wheel. Axial-entry dovetails may be straight (typically parallel to the wheel axis) or have a gradual curvature, the latter of which is represented in FIG. 1.
Axial-entry and in particular curved axial-entry dovetail designs are difficult to inspect using conventional ultrasonic pulse-echo techniques due to inadequate detection sensitivity caused by the blade attachment geometry, and particularly the surfaces of the female dovetail slots, which are roughly perpendicular to the fore and aft surfaces of turbine wheel following final machining. As a result, ultrasonic beams projected from these surfaces are not directed perpendicular to defects on the dovetail surfaces. As an alternative, dovetail inspections can be accomplished using pitch-catch techniques by placing transducers opposite each other on the fore and aft faces of a wheel. While capable of providing greater coverage for axial-entry dovetail slots, pitch-catch inspection systems are relatively difficult to set up (for example, transducer placement can be problematic) and are not comparable in sensitivity to pulse-echo inspections. Furthermore, the ultrasonic beams still do not intersect perpendicular to defects located at the interior dovetail surfaces. Regardless of the ultrasonic technique used, the ability to inspect female dovetail surfaces at the circumference of a wheel is exacerbated by the high sonic noise produced by large grain sizes typically found in wheels, as well as a tendency for the acoustic pulse to be steered by flow lines produced during the forging process.
In view of the above, it would be desirable if an improved ultrasonic inspection method were available that was capable of full ultrasonic coverage of surfaces of axial-entry dovetail slots located in the circumference of turbine wheels, as well as interior surfaces of other slots or slot-like features having complex geometries.